1. Field of the Invention
The field of the present invention relates to the preparation of thin film semiconductor devices. More particularly, the present invention relates to electrodeposition of copper-indium-gallium-diselenide films for solar cells.
2. Description of the Related Art
Chalcopyrite ternary thin films of copper-indium-diselenide (CuInSe.sub.2) and copper-indium-gallium-diselenide (CuIn.sub.1-x Ga.sub.x Se.sub.2), both of which are generically referred to as Cu(In,Ga)Se.sub.2, CIGS, or simply CIS, have become the subject of considerable interest and study for semiconductor devices in recent years. Sulphur can also be, and sometimes is, substituted for selenium, so the compound is sometimes also referred to even more generically as Cu(In,Ga)(Se,S).sub.2 so as to encompass all of these possible combinations. These devices are also referred to as I-III-VI.sub.2 devices according to their constituent elemental groups.
These devices are of particular interest for photovoltaic device or solar cell absorber applications. For photovoltaic applications, the p-type CIGS layer is combined with an n-type CdS layer to form a p-n heterojunction CdS/CIGS device. The direct energy gap of CIGS results in a large optical absorption coefficient, which in turn permits the use of thin layers on the order of 1-2 .mu.m. An additional advantage of CIGS devices is their long-term stability.
Various methods have been reported for fabricating CIGS thin films. Some of the earliest techniques involved heating copper and indium on a substrate in the presence of a selenium-containing gas, including H.sub.2 Se. The heating of copper and indium films in the presence of a selenium-containing gas is known as selenization. One drawback to selenizing with H.sub.2 Se is that H.sub.2 Se gas is highly toxic, thus presenting serious hazards to humans in large scale production environments.
In U.S. Pat. No. 5,045,409, Eberspacher et al. disclose depositing copper and indium films by magnetron sputtering, and depositing a selenium film by thermal evaporation, followed by heating in the presence of various gases. Other methods for producing CIS films have included Molecular Beam Epitaxy, electrodeposition either in single or multiple steps, and vapor deposition of single crystal and polycrystalline films.
Although vapor deposition techniques have been used to yield solar cells with efficiencies as high as seventeen percent (17%), vapor deposition is costly. Accordingly, solar cells made by vapor deposition have generally been limited to devices for laboratory experimentation, and are not suitable for large scale production. On the other hand, thin film solar cells made by electrodeposition techniques are generally much less expensive. However, solar cells produced by electrodeposition generally suffer from low efficiencies. For example, in Solar Cells with Improved Efficiency Based on Electrodeposited Copper Indium Diselenide Thin Films, ADVANCED MATERIALS, Vol. 6 No. 5 (1994), Guillemoles et al. report solar cells prepared by electrodeposition with efficiencies on the order of 5.6%.